Devtools: Await promises in inspector tests

A lot of our tests don't await promises which means they can rely on
ordering of tasks in the backend which is not guaranteed. Upcoming
work on the inspector task scheduler makes some of these tests
start to flake because they expect events to happen in a certain
order, even when they don't await those events.

These tests might already be flaky because of this but it's hard
to tell.

These aren't the only tests that don't await protocol messages but I
focused on tests that use Target.setAutoAttach or
runIfWaitingForDebugger as they trigger the flakiness.

Change-Id: I4709c47256ad7ff557a34d0def1721ceaa8af706
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936289
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719117}
12 files changed